Semiconductive metal oxides, for example ZnO, In2O3 and SnO2, are well known in the literature. They possess high charge carrier mobilities with simultaneous optical transparency. In commercial use, semiconductive metal oxides are currently being applied from the gas phase either by means of sputtering or other deposition methods. JP2007-073704 teaches formation of oxidic semiconductor components, in which various oxide layers are applied successively by means of sputtering.
Hong et al. (Thin Solid Films (2006) 515: 2717-2721), teach the repeated, successive application of individual thin oxide layers with one type of metal atom each by means of sputtering processes.
Song et al. (Solid State Communications (2010) 150: 1991-1994) teach the application of a zinc oxide layer on an indium oxide layer by means of metal-organic chemical gas phase deposition (MOCVD).
Kumomi et al. (Journal of Display Technology (2009) 5 (12):531-540) discuss the In—X—O system with various elements X. The In—X—O layer is applied by means of co-sputtering.
These methods are disadvantageous since they are technically complex and are associated with high costs in the purchase of the equipment used therefor.
For this reason, the literature is increasingly discussing liquid phase coating methods. In these processes, for example, oxide nanoparticles in a dispersion, or precursor molecules in a solvent, are used as liquid phases. Layers which are produced from the liquid phase differ, however, in several aspects from those which have been produced from the gas phase, since the use of solvents often causes residues of these solvent molecules to be incorporated into the layer. In addition, for example in the case of precursor solutions, the chemical conversion to the oxide is not always homogeneous.
A known problem in the case of semiconductive metal oxides with only one type of metal atom, irrespective of the process by which they have been produced, is inadequate stability both with respect to atmospheric influences and with respect to electrical stress, the latter being particularly commercially important.
A solution to this problem is the use of more complex metal oxides with two or more different types of metal atoms. In sputtering or vapour deposition technology, preference is therefore given to using systems such as indium gallium zinc oxide, since they are particularly stable. This approach has also been taken in liquid phase systems.
WO2010/122274 A1 teaches the formation of particularly stable components from solution by the introduction of alkali metal or alkaline earth metal atoms into the layer.
Kim et al. (APPLIED PHYSICS LETTERS (2010) 96: 163506) teach the liquid phase-based production of stable components by the incorporation of Mg metal atoms into the semiconductor layer.
US 2006-0088962 A1 teaches the application of oxidic dielectrics from solution to a semiconductor component for improvement of the semiconductor properties.
Nevertheless, in the known processes, particularly in the case of solutions or dispersions which have been optimized for optimal oxide layer formation at comparatively low conversion temperatures, it is not possible to directly add other metal atoms without having to accept disadvantages. Examples of disadvantages include a chemical reaction with the original solution, disruption of layer formation or an inhomogeneous stoichiometric ratio of the metal atoms on the surface. In these cases, the desired advantages with regard to electrical stability simultaneously entail disadvantages, for example a reduction in charge carrier mobility in the semiconductor layer.
The present invention overcomes the known disadvantages.